As conventional hot-water supply systems, there have been adopted a combustion-type hot-water supply system and an electric water heating system. The combustion-type hot-water supply system has no hot-water storage tank. In this unit, a gas is burned to instantly heat up water with its high combustion heat to supply hot water. On the other hand, the electric water heating system has a large-capacity hot-water storage tank, in which a large quantity of water heated during the nighttime with an electric water heater by utilizing a low-cost midnight power is held, so that the hot water held in the hot-water storage tank may be used during the daytime.
Recently, however, a heat pump hot-water supply system having a 300 to 500 percent higher energy efficiency than the electric water heating system has come into widespread use. The heat pump hot-water supply system, utilizing changes in the state of the refrigerant as a heat source, has several times higher energy efficiency than the electric water heater. Besides, since no gas combustion takes place, the hot-water supply system produces no CO2, thereby being valued as an eco-friendly system. However, because the quantity of heat produced in the heat pump hot-water supply system is not as great as compared with gas combustion, there is generally adopted such a method that a large-capacity hot-water storage tank is provided, like in the electric water heating system; the hot water heated with a nighttime low-cost electric power by the use of a heat pump circuit is held in a hot-water storage tank during the nighttime, and is used during the daytime. Therefore, in the conventional heat pump hot-water supply system, a heat pump circuit and a hot-water storage tank are separately mounted. The hot water produced by the heat pump circuit is voluminously held in the hot-water storage tank. The heat pump circuit and the hot-water storage tank are connected by a piping, to thereby combinedly function as a hot-water supply system.
There has been conventionally known a heat pump hot-water supply system as an example as disclosed in Japanese Patent Laid-Open No. 1997(H9)-126547. The heat pump hot-water supply system is comprised of a heat pump circuit including a compressor, a condenser, a pressure reducing unit, and an evaporator; and a large-capacity hot-water storage tank. In the unit, a water pipe is connected from under the hot-water storage tank to a water heat exchanger, which functions as the condenser of the heat pump circuit, via a circulating pump; and a heating circuit is constituted by connecting a hot-water supply pipe between the outlet port of the water heat exchanger and the upper part of the hot-water storage tank.
The high energy-efficiency heat pump circuit is operated by utilizing a low-cost nighttime electric power, to gradually heat up water to a predetermined temperature of hot water by circulating water in the hot-water storage tank by means of the circulating pump. Upon detecting by a temperature sensor that the water temperature has reached the predetermined hot water temperature, the operation of the heat pump circuit is stopped.
When using the hot water at a using end unit in the daytime, the hot water is supplied after lowering to an appropriate temperature by mixing, by a mixing valve, water and the hot water taken out at the upper part of the hot-water storage tank.
It is also described that, when the temperature of the hot water in the hot-water storage tank has dropped, the hot water in the hot-water storage tank is reheated at the water heat exchanger and the heater and supplied to the using end unit notwithstanding the hot water discharge channel is not described in detail.
However, considering that a bathtub is filled with the hot water from the hot-water storage tank, the required capacity of the hot-water storage tank is as close to 200 liters as the bathtub. Furthermore, where it is expected to allow a certain amount of room for use of the hot water for other purposes, the tank capacity in some cases will become as large as 500 liters. When the hot-water storage tank is filled up to its full capacity, the mass will exceed 200 kg to 500 kg. The heat pump hot-water supply system, therefore, needs the construction of a firm, substantially wide ground foundation for installation.
Furthermore, a great deal of energy is required to heat up a large quantity of hot water. In addition, storing water at high temperatures results in a substantial difference between the hot water temperature and the outside air temperature and therefore in a big heat loss by heat dissipation, consequently requiring the use of an excess energy.
The heat pump hot-water supply system, therefore, can not be installed in such a narrow space or a low-strength place as the balcony of an apartment house or a condominium.
The conventional heat pump hot-water supply system is basically operated by utilizing a nighttime low-cost electricity rate as follows. During the nighttime the heat pump circuit is operated to store the hot water in the hot-water storage tank. During the daytime the heat pump circuit is not operated and the hot water held in the hot-water storage tank is used. According to this unit, however, the hot water in the hot-water storage tank is sometimes used up; in such a case, water can not be heated instantly, resulting in hot water starvation. In the above-described heat pump hot-water supply system disclosed in Japanese Patent Laid-Open No. 1997(H9)-126547 also, the hot water is held by utilizing the nighttime electric power as in the case of the electric water heater, and additional heating is performed in the case of lack of hot water. The additional heating, therefore, is a supplementary operation.
Furthermore, heat is dissipated out from the surface of the hot-water storage tank which stores a large quantity of hot water of higher temperatures than the outside air temperature, resulting in waste of energy and a drop of temperatures. It is, therefore, necessary to heat an excess amount of water up to the same temperature as the temperature drop during the nighttime.